Conversion element and a light-emitting diode including such a conversion element

ABSTRACT

A conversion element for the wavelength conversion of electromagnetic radiation from a first wavelength range to electromagnetic radiation from a second wavelength range, which includes longer wavelengths than the first wavelength range, the conversion element includes: a matrix material, the optical refractive index of which is temperature-dependent, and at least two different types of luminophore particles wherein a multiplicity of luminophore particles of each of the types are distributed in the matrix material, luminophore particles of different types differ from one another in terms of average particle size and/or material, the conversion element, upon excitation by electromagnetic radiation from the first wavelength range emits mixed radiation including electromagnetic radiation from the first and the second wavelength range, and the correlated color temperature and/or the color locus of the mixed radiation remain(s) substantially the same when the matrix material is at a temperature of between 25° C. and 150° C.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/128,683, filed Dec. 23, 2013, which is a national stageentry according to 35 U.S.C. § 371 of PCT application No.:PCT/EP2012/058548 filed on May 9, 2012, which claims priority fromGerman application No.: 10 2011 078 402.0 filed on Jun. 30, 2011, and isincorporated herein by reference in its entirety.

TECHNICAL FIELD

A conversion element is specified.

SUMMARY

Various embodiments provide a conversion element having an improvedefficiency. Various embodiments also provide a conversion element whichcan be used to generate light with improved temperature stability of thecolor locus.

In accordance with at least one embodiment of the conversion element,the conversion element is suitable for converting electromagneticradiation from a first wavelength range to electromagnetic radiationfrom a second wavelength range, which includes longer wavelengths thanthe first wavelength range. That is to say that the conversion elementis provided for the so-called “down conversion” of electromagneticradiation. By way of example, the conversion element converts at leastpart of a primary radiation, for example blue light, to a secondaryradiation, for example red, green and/or yellow light. Overall, theconversion element can then emit mixed radiation composed of theelectromagnetic radiation from the first and second wavelength ranges.

The conversion element can be embodied as a self-supporting plate, whichmay be parallelepipedal, for example, as a flexible film or as pottingmaterial. The conversion element is suitable for being disposeddownstream of a radiation-emitting component, in particular alight-emitting diode chip. The conversion element then converts theelectromagnetic radiation from the first wavelength range generated bythe radiation-emitting component during operation at least partly toelectromagnetic radiation from the second wavelength range.

In accordance with at least one embodiment of the conversion element,the conversion element includes a matrix material. The matrix materialis a material which is transmissive to the electromagnetic radiationfrom the first wavelength range and electromagnetic radiation from thesecond wavelength range and which may be designed to be transparent forexample to electromagnetic radiation from these wavelength ranges. Thematrix material has an optical refractive index that istemperature-dependent. In particular, the refractive index istemperature-dependent in a temperature range of approximately 20° C. toapproximately 200° C. By way of example, the optical refractive indexdecreases with rising temperatures in this temperature range. The matrixmaterial may be a silicone or an epoxy resin, for example. Furthermore,it is possible for the matrix material to be a mixture composed ofsilicone and epoxy resin.

In accordance with at least one embodiment of the conversion element,the conversion element includes at least two different types ofluminophore particles, wherein a multiplicity of luminophore particlesof each of these types are distributed in the matrix material. Thematrix material surrounds the luminophore particles distributed in it,for example in a positively locking manner. That is to say that theluminophore particles may be embedded into the matrix material. In thiscase, it is possible for the luminophore particles of the differenttypes to be distributed randomly and as uniformly as possible in thematrix material. Furthermore, it is also possible for an accumulation ofluminophore particles to occur at specific locations of the matrixmaterial, said accumulation arising for example as a result of theluminophore particles sinking in the matrix material.

In accordance with at least one embodiment of the conversion element,luminophore particles of different types differ in terms of at least oneof the following properties: the average particle size and/or thematerial. In this case, it is also possible for the different types ofluminophore particles to differ from one another in terms of bothproperties. That is to say that the luminophore particles of differenttypes can be formed with different luminescence conversion materialsand/or have on average different sizes. As a result, the luminophoreparticles of different types have, for example, mutually differentemission wavelengths and/or different scattering behaviors.

In accordance with at least one embodiment of the conversion element,the conversion element, upon excitation by electromagnetic radiationfrom the first wavelength range, emits mixed radiation includingelectromagnetic radiation from the first and second wavelength ranges.That is to say that at least part of the electromagnetic radiation fromthe first wavelength range that enters into the conversion element isnot converted, but rather leaves the conversion element without beingconverted, such that mixed radiation including converted and unconvertedelectromagnetic radiation emerges from the conversion element. By way ofexample, said mixed radiation is white or colored light.

In accordance with at least one embodiment of the conversion element,the correlated color temperature and/or the color locus of the mixedradiation remain(s) substantially the same when the matrix material isat a temperature of between 20° C. and 200° C., in particular between25° C. and 150° C. In this case, “substantially the same” means, forexample, that an average observer of the mixed radiation, in the case ofa temperature change in the stated temperature range, cannot ascertainany change to the correlated color temperature and/or the color locuswith the naked eye. Therefore, to the observer the emitted mixedradiation appears to be constant with regard to cover temperature and/orcolor locus even in the case of temperature fluctuations.

In accordance with at least one embodiment of the conversion element,the conversion element includes a matrix material, the opticalrefractive index of which is temperature-dependent, and at least twodifferent types of luminophore particles, wherein a multiplicity ofluminophore particles of each of the types are distributed in the matrixmaterial, luminophore particles of different types differ from oneanother in terms of average particle size and/or material, theconversion element, upon excitation by electromagnetic radiation fromthe first wavelength range emits mixed radiation includingelectromagnetic radiation from the first and the second wavelengthrange, and the correlated color temperature and/or the color locus ofthe mixed radiation remain(s) substantially the same when the matrixmaterial is at a temperature of between 25° C. and 150° C.

A conversion element described here is in this case based on thefollowing considerations, inter alia:

During the operation of an optoelectronic component downstream of whicha conversion element is disposed, the temperature of the conversionelement changes as a result of the heating of the conversion element onaccount of waste heat from the optoelectronic component and/orconversion losses. The change in the temperature of the conversionelement generally leads to a change in the color locus and thus also toa change in the correlated color temperature of the mixed light emittedby the conversion element. This is undesirable for example when theconversion element is used in general lighting, since, as a result, thecolor impression of the generated light can change depending on ambienttemperature and depending on operating duration of the optoelectroniccomponent, which is perceived as unpleasant by the observer.

The shift in the color locus is caused by an interaction of manyphysical processes such as, for example:

-   -   the temperature-dependent shift of the electromagnetic radiation        from the first wavelength range generated by the optoelectronic        component,    -   the temperature dependence of the quantum efficiency of the        luminophore conversion,    -   the temperature dependence of the luminophore absorption,    -   the temperature dependence of the luminophore emission spectrum,    -   the temperature dependence of the light scattering in the        conversion element on account of the temperature dependence of        the refractive index of the matrix material of the conversion        element.

Particularly in the case of conversion elements which contain particlesof luminophores in the matrix material, the change in the refractiveindex of the matrix material with temperature plays an important part.By way of example, the refractive index decreases as the temperature ofthe matrix material rises, as a result of which the light scattering atthe luminophore particles introduced in the matrix material generallyincreases since the difference in refractive index between theluminophore particles and the matrix material increases. An increase inthe scattering leads to a lengthening of the average propagationdistance of the electromagnetic radiation from the first wavelengthrange in the conversion element and thus to an increase in theabsorption of electromagnetic radiation from the first wavelength range.In this way, therefore, the probability of conversion by luminophoreparticles of the conversion element increases, which leads to a colorlocus shift in the conversion direction.

Since the first three of the abovementioned processes very generallylead to a color locus shift in the direction of the electromagneticradiation from the first wavelength range, the change in refractiveindex brings about a partial compensation of this color locus shift. Theextent to which this compensation arises is dependent on the propertiesof the luminophore particles used, in particular on the refractive indexthereof and the particle size distribution, and on the matrix materialused. It has previously been established that the compensation isgenerally too small, however, to appreciably alter the shift in thedirection of the electromagnetic radiation from the first wavelengthrange, that is to say for example in the direction of blue.

One solution to this problem might consist in admixing with the matrixmaterial additional, non-converting scattering particles, composed of amaterial whose refractive index either at room temperature or at thetypical operating temperature is as close as possible to the refractiveindex of the matrix material. With utilization of thetemperature-dependent change in the refractive index of the matrixmaterial, these scattering particles then provide for additional orreduced scattering, depending on the temperature, and can stabilize thecolor locus of the emitted mixed radiation in this way. However, this isassociated with a reduction of the efficiency on account of additionalscattering losses at the scattering particles, at least at thetemperatures of the matrix material at which the refractive indices ofscattering particles and matrix material are not identical.

In the present case, therefore, it is proposed, inter alia, to use inthe conversion element a mixture of different luminophore types, themixture of which is optimized deliberately in such a way that astabilization of color temperature and/or color locus occurs over alarge temperature range. In this case, however, it should be taken intoconsideration that the refractive index of luminophore particles cannotbe chosen freely, since it is given by the material of the luminophoreparticles, the choice of which material is in turn determined by theintended emission wavelength of the luminophore particles.

A conversion element described here is based on the concept, inter alia,of mixing at least two luminophores of different types in such a waythat the color locus remains virtually constant in the case of a changein temperature in a relatively wide temperature range and/or thecorrelated color temperature does not change in a wide temperaturerange. This can be achieved by the use of at least two different typesof luminophores, wherein, in the case of one of the luminophoresdescribed, the compensation of the shift toward electromagneticradiation from the first wavelength range turns out to be too small and,in the case of the second type of luminophores, an overcompensationoccurs, that is to say that, for particles of this luminophore, themixed radiation would shift in the direction of the electromagneticradiation from the second wavelength range.

In accordance with at least one embodiment of the conversion element,the correlated color temperature of the mixed radiation varies by atmost 60 K when the matrix material is at a temperature of between 25° C.and 150° C. In other words, the correlated color temperature remainslargely stable in this temperature range of the matrix material.

In accordance with at least one embodiment of the conversion element,the color locus of the mixed radiation lies within a three-step McAdamellipse when the matrix material is at a temperature of between 25° C.and 150° C. In particular, it is possible for the color locus of themixed radiation to lie within a one-step McAdam ellipse in the statedtemperature range. In this case, the color locus shifts mentioned canhardly be perceived or cannot be perceived at all by the human observer.

In accordance with at least one embodiment of the conversion element,the refractive index of the matrix material decreases in a temperaturerange of between 25° C. and 150° C. with rising temperature of thematrix material. By way of example, this applies to a matrix materialwhich is formed with a silicone or consists of a silicone.

In accordance with at least one embodiment of the conversion element,the matrix material is substantially free of particles of a scatteringmaterial. In this case, a scattering material is understood to mean amaterial without wavelength-converting properties. “Substantially free”means that the concentration of particles of a scattering material,which particles can be present in the matrix material for example asresidues from the production of the luminophore particles, is at most0.1% by weight relative to the total weight of the conversion element.In other words, a scattering material is not deliberately introducedinto the matrix material of the conversion element.

In accordance with at least one embodiment of the conversion element,the conversion element includes three types of luminophore particles,wherein two types of luminophore particles have an identical or similarfirst emission wavelength range. In other words, two types ofluminophore particles have an emission wavelength range in which themaximum of the emission wavelength differs from one another by less than60 nm, in particular by less than 50 nm. The two types of luminophoreparticles then re-emit light of the same color, in particular. Uponobservation of the dominant wavelength, the difference between theemission maxima of the two types of luminophore particles is preferablyat most 30 nm, in particular at most 20 nm. Furthermore, it is possiblefor the luminophore particles of different types to be formed with thesame material and therefore to have an identical emission wavelengthrange.

In accordance with this embodiment, the conversion element may include athird type of luminophore particles, which has a further emissionwavelength range, the maximum of which deviates from the maximum of thefirst emission wavelength range by at least 100 nm, in particular by atleast 150 nm.

In other words, the luminophore particles of the third type emit lightof a different color than the luminophore particles of the first andsecond types.

In accordance with at least one embodiment of the conversion element,the luminophore particles of at least two of the different types consistof different materials and have a similar emission wavelength range.

By way of example, the conversion element may include three types ofluminophore particles composed of the following materials: LuAGaG:Ce asluminophore particles which re-emit green light, (Sr,Ca,Ba)₂Si₅N₈ asluminophore particles which emit red light, and CaAlSiN likewise asluminophore particles which emit red light. For the luminophoreparticles mentioned last, the overcompensation described above arises,while the luminophore particles mentioned second do not enablesufficient compensation of the color locus shift by a change of thescattering. In the mixture of the three stated types of luminophoreparticles in the common matrix material, which is formed with silicone,for example, this results in a virtually constant color temperature inthe abovementioned temperature range.

In accordance with at least one embodiment of the conversion element,the luminophore particles of at least two of the different types ofluminophore particles are formed from the same material and differ withregard to their average particle size. In this case, the scatteringeffect at different temperatures is determined by the different particlesizes. The mixture of luminophore particles having different averageparticle sizes makes it possible to deliberately set the scatteringproperties for electromagnetic radiation from the first wavelength rangewhich enters into the conversion element.

A light-emitting diode is furthermore specified. A conversion elementdescribed here can be used in the light-emitting diode. That is to saythat all features described for the conversion element are alsodescribed for the light-emitting diode.

In accordance with at least one embodiment of the light-emitting diode,the light-emitting diode includes at least one light-emitting diode chipwhich emits electromagnetic radiation from the first wavelength rangeduring operation. This involves blue light, for example. Furthermore,the light-emitting diode includes a conversion element described here,which is disposed downstream of the light-emitting diode chip in such away that at least part of the electromagnetic radiation from the firstwavelength range passes into the conversion element and is converted toelectromagnetic radiation from the second wavelength range. Theelectromagnetic radiation from the two wavelength ranges then mixes toform mixed radiation, which can be white light, for example. In thiscase, the conversion element may be applied as a thin layer to aradiation exit surface of the light-emitting diode chip. Furthermore, itis possible for a prefabricated conversion element, for example in theform of a lamina or a film, to be applied to the radiation exit surfaceof the light-emitting diode chip. Furthermore, it is possible for theconversion element to envelop the light-emitting diode chip in apositively locking manner. In this case, the conversion element isapplied to the light-emitting diode chip for example in the manner of apotting material by injection molding or transfer molding.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. The drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the disclosed embodiments. In the following description,various embodiments described with reference to the following drawings,in which:

FIG. 1 shows an embodiment of a conversion element described here on thebasis of a schematic sectional illustration.

FIG. 2 shows an embodiment of a light-emitting diode described here onthe basis of a schematic sectional illustration.

FIGS. 3 and 4 show properties of conversion elements and light-emittingdiodes described here on the basis of graphical plots.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingthat show, by way of illustration, specific details and embodiments inwhich the disclosure may be practiced.

The schematic sectional illustration in FIG. 1 schematically illustratesa conversion element 1 described here. The conversion element 1 includesa matrix material 3, which is formed of a silicone, for example. In atemperature range of at least 25° C. to at most 150° C., for example,the optical refractive index of the matrix material 3 decreases withrising temperature.

A multiplicity of luminophore particles are introduced into the matrixmaterial 3. In this case, into the matrix material 3 in the embodimentin FIG. 3, three different types of luminophore particles are introducedinto the matrix material 3. By way of example, the conversion element 1includes the following types of luminophore particles: luminophoreparticles of first type 2 a, which are formed with LuAGaG:Ce,luminophore particles of second type 2 b, which are formed with(Sr,Ca,Ba)₂Si₅N₈, and luminophore particles of third type 2 c, which areformed with CaAlSiN.

Electromagnetic radiation 10 from a first wavelength range which entersinto the conversion element 1 is partly wavelength-converted by theluminophore particles, wherein electromagnetic radiation from a secondwavelength range 10 a, 10 b, 10 c is generated. The latter radiationmixes with the electromagnetic radiation from the first wavelength range10 to form mixed radiation 11, which can be white light, for example. Inthe present case, the luminophore particles of the first type 2 a emitgreen light 10 a, the luminophore particles of the second type 2 b emitred light 10 b and the luminophore particles of the third type 2 clikewise emit red light 10 c. In the present case, the luminophoreparticles of the second type and third type are formed from differentmaterials, but have a similar emission wavelength, that is to say thatthe maximum of the emission wavelength of both types of luminophoreparticles differs by less than 50 nm.

Color loci for different conversion elements at different temperaturesof the matrix material are plotted in conjunction with FIG. 3.

The starting point is chosen at 25° C. and represented by the thicksymbols of the curves 22, 23 and 24 in the graphical plot in FIG. 3. TheJudd straight line 21 is entered in FIG. 3 in a dashed fashion.

The curve 22 relates to a first luminophore mixture including thefollowing luminophores: (Sr,Ca,Ba)₂Si₅N₈, LuAGaG:Ce. The curve 23relates to a second luminophore mixture including the followingluminophores: LuAGaG:Ce, CaAlSiN. The curve 24 relates to a thirdluminophore mixture including the following luminophores LuAGaG:Ce,CaAlSiN, (Sr,Ca,Ba)₂Si₅N₈, that is to say the luminophores of the firstand second luminophore mixtures. In the luminophore mixture mentionedlast, the two red luminophores are introduced for example in a ratio of1:1.

In this case, the individual measurement points of the curves 22, 23 and24 are each spaced apart from one another by 25° C. In this case, theconcentrations of the individual luminophore particles are set in such away that approximately the same color locus is attained by all theluminophore mixtures at approximately 125° C. The curve 21 representsthe Judd straight line for 3000 K, showing the color loci with identicalmost similar color temperature. This line is simultaneously the majoraxis of a McAdams ellipse. A change in color locus along this line isvirtually imperceptible to the human eye. If a mixture of luminophoresincluding the three stated types of luminophore particles, curve 24, isused, wherein the red luminophores are present in a mixture ofapproximately 1:1, then the color locus of the mixed light, 11, runsapproximately along the Judd straight line 21 in the range between 25°C. and 150° C.

FIG. 4 shows a graphical plot in which the color temperature is plottedas a function of the temperature of the matrix material for the threeluminophore mixtures. It is evident from this illustration that thevariation of the color temperature between 25° C. and 150° C. is lessthan 50 K for the curve 24.

The schematic sectional illustration in FIG. 2 illustrates alight-emitting diode including a conversion element 1 described here.The light-emitting diode includes a carrier 4, which is, for example, aconnection carrier such as a leadframe or a printed circuit board onwhich a light-emitting diode chip 5 that emits blue light is applied.The light-emitting diode chip 5 is enveloped by the conversion element 1in a positively locking manner, for example plotted therewith.

While the disclosed embodiments have been particularly shown anddescribed with reference to specific embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the disclosed embodiments as defined by the appended claims. Thescope of the disclosed embodiments is thus indicated by the appendedclaims and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced.

What is claimed is:
 1. A method for producing a conversion elementcomprising: choosing a matrix material; distributing at least twodifferent types of luminophore particles in the material matrix; whereinthe at least two different luminophore particles comprise at least(Sr,Ca,Ba)₂Si₅N₈ and CaAlSiN in a 1:1 ratio; wherein the correlatedcolor temperature and the color locus of the electromagnetic radiationemitted from the at least two different luminophore particles aresubstantially the same at a temperature between at least 25° C. and atmost 150° C.; distributing the multiplicity of second luminophoreparticles in the material matrix.
 2. The method as claimed in claim 1,wherein a multiplicity of second luminophore particles are chosen suchthat correlated color temperature of the mixed radiation varies by atmost 60 K when the matrix material is at a temperature of between atleast 25° C. and at most 150° C.
 3. The method as claimed in claim 2,wherein the multiplicity of second luminophore particles are chosen suchthat color locus of the mixed radiation lies within a three-step McAdamellipse when the matrix material is at a temperature of between at least25° C. and at most 150° C.
 4. The method as claimed in claim 1, whereinmatrix material is chosen such that the refractive index decreases withrising temperature in the temperature range of between at least 25° C.and at most 150° C.
 5. The method as claimed in claim 1, wherein thematrix material is chosen such that the matrix material is substantiallyfree of particles of a scattering material.
 6. The method as claimed inclaim 1, further comprising: choosing a multiplicity of thirdluminophore particles; wherein second luminophore particles are chosento have an identical or a similar first emission wavelength range tofirst luminophore particles and the third luminophore particles have afurther emission wavelength range, the maximum of which deviates fromthe maximum of the first emission wavelength range by at least 100 nm.7. The method as claimed in claim 6, wherein the luminophore particlesof at least two of the different multiplicities consist of differentmaterials and have a similar emission wavelength range.
 8. The method asclaimed in claim 6, further comprising luminophore particles composed ofLuAGaG:Ce, luminophore particles composed of (Sr,Ca,Ba)2Si5N8 andluminophore particles composed of CaAlSiN.
 9. The method as claimed inclaim 6, wherein the luminophore particles of at least two of thedifferent multiplicities consist of the same material and have differentaverage particle sizes.
 10. The method as claimed in claim 1, envelopinga light-emitting diode chip in a positively locking manner.
 11. A methodfor producing a conversion element comprising: choosing a matrixmaterial; distributing at least two different types of red luminophoreparticles in the material matrix; wherein the two different types of redluminophore particles comprise at least two different types of redluminophores in a 1:1 ratio; wherein the at least two different types ofred luminophores are not rare earth elements; wherein the correlatedcolor temperature and the color locus of the electromagnetic radiationemitted from the at least two different luminophore particles aresubstantially the same at a temperature of between at least 25° C. andat most 150° C.; distributing the multiplicity of second luminophoreparticles in the material matrix.